Author: Charles Platt / Source: Make: DIY Projects and Ideas for Makers
Forty years ago, the SN76477 was born: a legendary chip designed by Texas Instruments to create sound effects in arcade games. If you’ve ever played Space Invaders, you’ve listened to the SN76477.
In those primitive times, controlling this chip was a challenge for hobbyists.
You could use rotary switches and pushbuttons (as I’ll suggest below), but if you got ambitious and hoped, maybe, you could run it from the cassette port on an Apple II — well, maybe not.Everything is now much easier, because many functions of the SN76477 are set via logic inputs. Just send high or low signals from an Arduino or any other 5V microcontroller, and you can sequence a cacophony of rifle shots, sirens, the puffing of a steam locomotive, and a sentimental lullaby. Better still, all of the sounds will share that retro arcade ambience.
Despite the age and obsolescence of the SN76477, you can still buy samples from multiple eBay suppliers for around $15. Apply a 9V battery, and the chip contains its own voltage regulator which converts the power to 5V internally and also makes it available (up to 10mA) from a 5V output. This you can use to power the logic inputs.
Sound from the chip has to be amplified, and the manufacturer recommends using a couple of transistors. I found that a single 2N3904 would work.
A scan of the original datasheet is kindly maintained by Experimentalists Anonymous at experimentalistsanonymous. Before playing with the SN76477, you should download a copy.
Careful study of this seminal document showed me that the chip synthesizes sound from three sources inside itself, as shown above. A voltage-controlled oscillator (VCO) creates tones that vary in pitch with voltage, and a super-low frequency oscillator (SLF) can control the VCO, creating the irritating whoop-whoop or weoo-weoo-weoo sounds that will induce paroxysms of nostalgia in arcade dwellers of the 1980s. An additional noise generator creates white noise, which is useful to simulate explosions.A mixer can blend any two or all three of the sources, and an envelope generator can modify the attack and decay of one-shot sounds such as a bird chirping.
The mixer merges sounds by ANDing them. If you want to hear sounds that can be distinguished from each other even though they occur simultaneously (for instance, bird song during a nuclear explosion) you have to alternate the mixer inputs at a rate of around 50kHz. This can be done with a 555 timer and a multiplexer.
All the sound attributes are adjustable with external resistors and capacitors. If you prefer to select capacitors using a microcontroller rather than rotary switches, you’ll need more multiplexers. I don’t have space to get into that topic here, but if you have my…
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